1. Field of the Invention
The present invention relates to improvements in an electric power steering device.
2. Description of the Related Art:
In recent years, an electric power steering device has been used extensively to as the muscular effort or force required to steer or turn the steering handle can be reduced and hence provides a comfortable steering touch to the driver. The electric power steering device of the type concerned is constructed such that an auxiliary torque generated by an electric motor in proportion to the steering torque is transmitted via a mechanical clutch to the steering system. One example of such electric power steering device is disclosed in Japanese Patent Laid-open Publication No. SHO 64-69829, entitled "Clutch Device".
The disclosed electric power steering device includes a cylindrical outer member connected to an electric motor, a shaft including a polygonal inner member connected to a steering wheel, and a clutch device for selectively connecting the internal member and the external member. The clutch device is composed of a plurality of friction clutch mechanisms disposed on the same circle. The clutch mechanisms each include a pair of wedge-shaped spaces (tapering spaces) defined between the inside surface of the cylindrical outer member and the outside surface of the polygonal inner member, a pair of rollers disposed in the wedge-shaped spaces, respectively, a spring acting between the rollers, and a retainer for positioning the rollers. The rollers selectively engage and disengage the outer member and the inner member (to engage and disengage the clutch device) in response to the movement of the retainer in a circumferential direction.
In general, at the initial stage of a steering motion of the steering wheel (caused when the steering wheel is turned back from one direction to the opposite direction, for example), only a very small steering torque is developed and, hence, the motor is not activated. Thus, no auxiliary torque is transmitted to the clutch device. Accordingly, in order to disengage the clutch device, a force which is greater than a frictional force required to lock up the rollers must be exerted. This clutch-releasing force should preferably be as small as possible so that the driver can get a comfortable steering touch.
On the other hand, when the need arises due to some reasons, the clutch device must be smoothly disengageable even though transmission of the auxiliary torque from the motor to the steering system is still continuing. In this case, clutch releasing operation requires a greater clutch-releasing force than as usual, posing a greater burden to the driver.